Choice A is the correct answer. Use the half-life concept.

1. Half-Life: Every 10 days, the amount is multiplied by $0.5$.
2. After 10 Days: $800 \times 0.5 = 400$ grams.
3. Formula: $A(t) = 800(0.5)^{t/10}$ where $t$ is in days.

💡 Strategic Tip: After one half-life period, exactly half remains.

Choice B is incorrect because this would be after 20 days (two half-lives). Choice C is incorrect because this is not half of 800. Choice D is incorrect because this would be after 30 days.